Hub moves, focus flips and item selects now flow through the rail focus
model + ListenableSelectors (two headers/cards repaint instead of every
visible card rebuilding). DiscoverScreen memoizes the hubs list, gates
provider notifies on a render signature and caches the rail widget so
no-op refreshes skip the subtree entirely. The whole-rail 0.6 and
inactive-row 0.7 AnimatedOpacity layers are replaced by AnimatedDimScrim
quads, removing the permanent per-frame saveLayers that kept low-end TV
GPUs raster-bound.
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